Continuous Observational Data of Atmospheric CH4 Mixing Ratios at Cape Ochi-ishi

The present data set is time series data of atmospheric methane (CH4) mixing ratios measured at Cape Ochiishi situated at the base of Nemuro Peninsula in eastern Hokkaido, Japan. The monitoring station is located on top of a cliff 42.4 m above ground at the southern tip of the cape facing the Pacific Ocean. Air is sampled from an intake placed on a tower adjacent to the observation station about 51.5 m above ground level, and the CH4 mixing ratios are measured semi-continuously by using a gas chromatographic system. The winds tend to be predominantly northwesterly in winter and southwesterly in summer, whereas in spring and autumn no predominant wind direction has been observed. Because Hokkaido’s eastern part is dotted with swamps and grazing lands which are methane sources, and two cities, Nemuro and Kushiro are situated 20 km north and 100 km west, respectively, of the station, the influence from local methane emission sources is frequently observed.